A photonic integrated circuit (PIC) is useful as an optical data link in applications such as optical communications, high performance computing, and data centers. For mobile computing platforms too, a PIC is a promising I/O for rapidly updating or syncing a mobile device with a host device and/or cloud service where a wireless link has insufficient bandwidth. Such optical links utilize an optical I/O interface that includes an optical transmitter and/or an optical receiver, at least one of which utilizes a photodetector, typically a photodiode.
An Avalanche Photodiode (APD) is one type of photodiode that is particularly useful in applications where high sensitivity is desired because they can offer signal gain through carrier multiplication mechanisms within the photoelectric semiconductor material. Such applications include fiber-optic telecommunication, laser rangefinders, and single photon level detection and imaging, among other applications. The gain-bandwidth product is a key metric in photodetectors and some of the most promising APD designs have employed silicon because one important property limiting the gain-bandwidth product is the effective k ratio (keff) of the photoelectric material. The keff is a ratio between hole and electron impact ionization coefficients, and a low keff is desirable for an APD. Silicon has an excellent keff of <0.1, however it suffers low absorbance in the near infrared band utilized by many photonic applications (e.g., fiber-optic telecom). Germanium and many III-V material systems do have good responsivity at such wavelengths, however keff in these materials is so high that they have thus far proven unsuitable for APD applications. For example, keff of InP is 0.4-0.5 and keff of Ge is 0.8-0.9. Furthermore, it is costly and technically difficult to monolithically integrate Ge or III-V materials on a silicon substrate. For example, epitaxial processing is often needed, which is expensive.
Another issue limiting the performance of photodetectors, and an APD in particular, is high dark current. High dark current, like excess noise, can limit sensitivity of a detector. While dark currents in the nanoamp range are often achieved in a silicon-based APD, dark currents in Ge-based APDs, for example, can be in the tens or hundreds of microamps. Dark current can have a number of sources, including thermionic emission and trap-assisted tunneling resulting from Fermi-level pinned surface states and crystal defects, for example stemming from lattice mismatch (e.g., 4% mismatch between Ge and Si).
High operating biases also remain an obstacle to a PIC integrating silicon CMOS circuitry and photonics onto a single chip. Silicon-based electrical circuitry, such as analog circuitry for sensing photodiode output, is typically designed for a 3.3V supply. APDs described in the art however often require a significantly higher bias voltage and are therefore generally beyond the operating space of even system-on-chip (SoC) technologies.
A practical photodetector design and a fabrication process permitting low voltage operation with sufficient gain-bandwidth product in the near infrared wavelength would therefore be advantageous.